Use Taulman t-glase and after a light sanding with really fine paper (optional really, but go for it if you can), spray it with polyurethane varnish or something similar. Check out the article here.

Are there any ways to make a 3D print transparent?

I am aware of several "clear" filaments for a ABS or PLA printer. They, however, have a cloudy or frosted glass appearance. I do not believe this is possible to eliminate but I believe it can be reduced.

Are there effective ways to make a print have a more transparent appearance?